lacp: Make lacp negotiation test hard-code aggregation keys.

The lacp implementation takes the aggregation key from the key or portid
of the first slave to be added to the lacp object.  When multiple slaves
are added initially to a lacp object (the most common case), which is the
"first" is arbitrary.  Until now, it seems that the "first" was actually
predictable enough for the tests to (unknowingly) rely on it, but an
upcoming commit will break that.  This commit fixes the test by forcing
a particular aggregation key for each lacp object.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>